Patterning methodology for uniformity control

ABSTRACT

The present disclosure provides a method of fabricating a semiconductor device. The method includes forming a patternable layer over a substrate. The method includes forming a first layer over the patternable layer. The method includes forming a second layer over the first layer. The second layer is substantially thinner than the first layer. The method includes patterning the second layer with a photoresist material through a first etching process to form a patterned second layer. The method includes patterning the first layer with the patterned second layer through a second etching process to form a patterned first layer. The first and second layers have substantially different etching rates during the second etching process. The method includes patterning the patternable layer with the patterned first layer through a third etching process.

BACKGROUND

The semiconductor integrated circuit (IC) industry has experienced rapidgrowth. Technological advances in IC materials and design have producedgenerations of ICs where each generation has smaller and more complexcircuits than the previous generation. However, these advances haveincreased the complexity of processing and manufacturing ICs and, forthese advances to be realized, similar developments in IC processing andmanufacturing are needed. In the course of IC evolution, functionaldensity (i.e., the number of interconnected devices per chip area) hasgenerally increased while geometry size (i.e., the smallest componentthat can be created using a fabrication process) has decreased.

The decreased geometry size leads to challenges in fabrication. Forexample, as geometry sizes continue to decrease, it is more difficult toachieve critical dimension (CD) uniformity for semiconductor devices.For example, poor CD uniformity may be a result of variations intopography. Poor CD uniformity may lead to undesirable drifting of draincurrents and threshold voltages of transistors. Traditionally, whengeometry sizes are relatively large, the topography variations may havea negligible effect on the CD uniformity. However, as geometry sizesbecome smaller and smaller, even slight variations in topography mayhave a detrimental effect on CD uniformity. Furthermore, to asemiconductor foundry, it may need to interact with multiple customerswhose devices each have their own unique topography. Consequently, theCD uniformity issue may be more pronounced for the semiconductorfoundry.

Traditional fabrication method of controlling CD uniformity areexpensive and tend to suffer from undesired lateral etching problemswhich may limit the effectiveness of the CD uniformity control.Therefore, while traditional methods of CD uniformity control have beengenerally adequate for their intended purposes, they have not beenentirely satisfactory in every aspect.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isemphasized that, in accordance with the standard practice in theindustry, various features are not drawn to scale. In fact, thedimensions of the various features may be arbitrarily increased orreduced for clarity of discussion.

FIG. 1 is a flowchart illustrating a method of controlling criticaldimension uniformity of a semiconductor device according to variousaspects of the present disclosure.

FIGS. 2-10 are diagrammatic fragmentary cross-sectional side views of asemiconductor device at various stages of fabrication in accordance withvarious aspects of the present disclosure.

DETAILED DESCRIPTION

It is understood that the following disclosure provides many differentembodiments, or examples, for implementing different features of variousembodiments. Specific examples of components and arrangements aredescribed below to simplify the present disclosure. These are, ofcourse, merely examples and are not intended to be limiting. Forexample, the formation of a first feature over or on a second feature inthe description that follows may include embodiments in which the firstand second features are formed in direct contact, and may also includeembodiments in which additional features may be formed between the firstand second features, such that the first and second features may not bein direct contact. In addition, the present disclosure may repeatreference numerals and/or letters in the various examples. Thisrepetition is for the purpose of simplicity and clarity and does not initself dictate a relationship between the various embodiments and/orconfigurations discussed.

FIG. 1 is a flowchart of a method 20 for fabricating a semiconductordevice. The method 20 begins with block 22 in which a patternable layeris formed over a substrate. The method 20 continues with block 24 inwhich a first layer is formed over the patternable layer. The method 20continues with block 26 in which a second layer is formed over the firstlayer. The second layer is substantially thinner than the first layer.The method 20 continues with block 28 in which the second layer ispatterned with a photoresist material through a first etching process toform a patterned second layer. The method 20 continues with block 30 inwhich the first layer is patterned with the patterned second layerthrough a second etching process to form a patterned first layer. Thefirst and second layers have substantially different etching ratesduring the second etching process. The method 20 continues with block 32in which the patternable layer is patterned with the patterned firstlayer through a third etching process.

FIGS. 2-10 are diagrammatic fragmentary cross-sectional side views of asemiconductor device 40 at various stages of fabrication in accordancewith various aspects of the present disclosure. It is understood thatFIGS. 2-10 have been simplified for a better understanding of theinventive concepts of the present disclosure. Accordingly, it should benoted that additional processes may be provided before, during, andafter the processes shown in FIGS. 2-10, and that some other processesmay only be briefly described herein.

Referring to FIG. 2, the semiconductor device 40 is a semiconductor ICchip, of which only a portion is shown here. The semiconductor device 40includes a substrate 45. In the embodiment shown, the substrate 45 is asilicon substrate that is doped with a P-type dopant such as boron. Inanother embodiment, the substrate 45 is a silicon substrate that isdoped with an N-type dopant such as arsenic or phosphorous.

Isolation structures such as isolation structure 50 are formed in thesubstrate 45. The isolation structure 50 includes a shallow trenchisolation (STI) device. The STI device has a dielectric material, whichmay be silicon oxide or silicon nitride. The STI device is formed byetching a trench in the substrate 45 and thereafter filling the trenchwith the dielectric material. The formation of the isolation structure50 may result in unevenness or variations of the topography on thesurface of the substrate 45. In addition, the topography on the surfaceof the substrate 45 may vary for other reasons, such as imperfectionsrelated to other fabrication processes, and/or variations associatedwith different product patterns from different customers. For the sakeof simplicity, these variations in the topography of the substratesurface are not explicitly shown in FIG. 2 or the subsequent figures,but it is understood that these topography variations exist.

Doped wells such as doped wells 60 and 61 are also formed in thesubstrate 45. The doped wells 60-61 are formed on either side of theisolation structure 50. In the embodiment shown in FIG. 2, the dopedwells 60-61 are doped with an N-type dopant such as arsenic orphosphorous. In another embodiment, the doped wells 60-61 may be dopedwith a P-type dopant. The doping may be carried out using an ionimplantation process known in the art.

A high-k dielectric layer 70 is formed over the substrate 45. The high-kdielectric layer 70 is formed using a deposition process known in theart, for example by an atomic layer deposition (ALD) process. The high-kdielectric layer 70 includes a high-k dielectric material. A high-kdielectric material is a material having a dielectric constant that isgreater than a dielectric constant of SiO₂, which is approximately 4.For example, the high-k dielectric material may include hafnium oxide(HfO₂), which has a dielectric constant that is in a range fromapproximately 18 to approximately 40. Alternatively, the high-k materialmay include one of ZrO₂, Y₂O₃, La₂O₅, Gd₂O₅, TiO₂, Ta₂O₅, HfErO, HfLaO,HfYO, HfGdO, HfAlO, HfZrO, HfTiO, HfTaO, SrTiO, or combinations thereof.

The high-k dielectric layer 70 will serve as a gate dielectric for atransistor to be formed later. In an embodiment, the high-k dielectriclayer 70 has a thickness that is in a range from about 5 angstroms toabout 30 angstroms. It is understood that the thickness of thedielectric layer 70 may be in a different range in alternativeembodiments.

A conductive layer 80 is formed over the high-k dielectric layer 70. Theconductive layer 80 is formed by a deposition process known in the artsuch as an ALD process. The conductive layer 80 may serve as a cappinglayer for the high-k dielectric layer 70. In an embodiment, theconductive layer 80 includes a metal material, for example titaniumnitride, and has a thickness that is in a range from about 20 angstromsto about 60 angstroms. It is understood that the thickness of theconductive layer 80 may be in a different range in alternativeembodiments.

A polysilicon layer 90 is formed over the conductive layer 80. Thepolysilicon layer 90 is formed by a deposition process known in the art,such as an ALD process, a chemical vapor deposition (CVD) process, aphysical vapor deposition (PVD) process, or combinations thereof. Thepolysilicon layer 90 will be patterned later into different polysiliconportions, where each polysilicon portion will serve as a dummy gateelectrode in a gate-last or gate-replacement process. This will bediscussed in more detail later. In an embodiment, the polysilicon layer90 has a thickness 95 that is in a range from about 700 angstroms toabout 1200 angstroms.

A hard mask layer 100 is formed over the polysilicon layer 90. The hardmask layer 100 will serve as a mask for the polysilicon layer 90 belowwhen the polysilicon layer 90 is etched into dummy polysilicon gateelectrodes in the later process. In an embodiment, the hard mask layer100 includes a silicon nitride layer and a silicon oxide layer formedover the silicon nitride layer. Both the silicon nitride layer and thesilicon oxide layer can be formed by CVD, PVD, ALD, or combinationsthereof. The silicon nitride layer has a thickness that is in a rangefrom about 50 angstroms to about 150 angstroms. The silicon oxide layerhas a thickness that is in a range from about 700 angstroms to about1100 angstroms. Thus, in that embodiment, the hard mask layer 100 has athickness 105 (which is the sum of the thicknesses of the siliconnitride layer and the silicon oxide layer) that is in a range from about750 angstroms to about 1250 angstroms. It is understood that thethickness of the hard mask layer 100 may be in a different range inalternative embodiments.

An anti-reflective layer 110 is formed over the hard mask layer 100. Theanti-reflective layer 110 may also be referred to as a bottomanti-reflective coating (BARC) layer. The anti-reflective layer 110 isformed by a spin-coating process known in the art. In an embodiment, theanti-reflective layer 110 has a thickness 115 that is in a range fromabout 1000 angstroms to about 2000 angstroms, which is much thicker thanother anti-reflective layers used in traditional semiconductorfabrication processes.

The anti-reflective layer 110 may be a naphthalene type or Aromatic typepolymer. The anti-reflective layer 110 has a relatively high carbonconcentration and a relatively low hydrogen concentration (both measuredin terms of weight). The carbon concentration is at least an order ofmagnitude (10 times) greater than the hydrogen concentration. Theanti-reflective layer 110 also has a relatively high refractive index(N) and a relatively low extinction coefficient (K). The refractiveindex may be greater than about 1, and the extinction coefficient may belower than about 0.5.

In one embodiment, the anti-reflective layer 110 includes a materialthat has a carbon concentration in a range from about 65% to about 70%,a hydrogen concentration in a range from about 5.5% to about 6.5%, amolecular weight in a range from about 3500 mw to about 4500 mw, arefractive index in a range from about 1.6 to about 1.7, and anextinction coefficient in a range from about 0.2 to about 0.3, whereboth the refractive index and the extinction coefficient are measuredwith respect to a 193 nanometer (nm) fabrication process.

In another embodiment, the anti-reflective layer 110 includes a materialthat has a carbon concentration in a range from about 80% to about 85%,a hydrogen concentration in a range from about 3% to about 5%, amolecular weight in a range from about 4500 mw to about 5500 mw, arefractive index in a range from about 1.4 to about 1.6, and anextinction coefficient in a range from about 0.3 to about 0.4, whereboth the refractive index and the extinction coefficient are measuredwith respect to the 193 nm fabrication process.

In other alternative embodiments, the anti-reflective layer 110 mayinclude I-line photoresist or some other type of photoresist.

The anti-reflective layer 110 serves to reduce the reflection of lightin a patterning process, for example through absorption. The reductionof reflected light helps improves the patterning performance. Accordingto the present disclosure, the anti-reflective layer 110 is also formedto be thick for purposes of diluting topography variation effects. Asdiscussed earlier, due to various fabrication process imperfections aswell as different product patterns from different customers, the surfacetopography of the substrate 45 may be uneven and may have variousconcave and convex portions. For example, one region of the substrate 45(such as the isolation structure 50) may protrude outward and be higherthan the rest of the substrate 45. The various layers 70-100 formed overthe substrate 45 thereafter are formed in a conformal manner. Hence, thetopography variations of the substrate 45 will be “inherited” by thelayers 70-100 formed above, which will also exhibit similar topographyvariations. These topography variations may result in critical dimension(CD) uniformity issues later on, and as such are undesirable.

Here, the anti-reflective layer 110 is formed to be thick so as toreduce the harmful effects of the topography variations. This is becausethe topography variations become smaller in relative terms as theanti-reflective layer 110 becomes thicker. For example, one type oftopography variation is a step height, which measures the amount ofprotrusion of an STI device (for example the STI device of the isolationstructure 50) over a substrate. For the sake of providing an example, atypical step height may be about 100 angstroms. A traditionalanti-reflective layer may have a thickness that is about 400-500angstroms. Therefore, a ratio range of the step height to thetraditional anti-reflective layer is from about 1:4 to about 1:5.

Here, the thickness 115 of the anti-reflective layer 110 is in a rangefrom about 1000 to about 2000 angstroms. Accordingly, the ratio of thestep height to the anti-reflective layer 110 is from about 1:10 to about1:20, which is much better than the traditional ratio range. Therelative reduction of the step height with respect to theanti-reflective layer 110 means that the harmful effects caused bytopography variations are diluted. And since the anti-reflective layer110 is formed by a spin-coating process—which is not a conformalprocess—its top surface is substantially flat. So future layers formedover the anti-reflective layer 110 will not have substantial topographyvariations.

A sacrificial layer 120 is formed over the anti-reflective layer 110.The sacrificial layer 120 may also be referred to as a middle layer. Thesacrificial layer 120 is formed by CVD, PVD, ALD, or combinationsthereof. The sacrificial layer 120 has a thickness 125. In oneembodiment, the sacrificial layer 120 includes a thin dielectricmaterial. The dielectric material may be Tetraethyl orthosilicate (TEOS)and may contain some organic material. The dielectric material has to bethin enough for proper exposure during photolithography. Thus, thethickness 125 of the sacrificial layer 120 is in a range from about 50angstroms to about 200 angstroms when it includes dielectric.

In another embodiment, the sacrificial layer 120 includes an organicmaterial (such as a photoresist material) that is doped with nitrogen.In that case, the sacrificial layer 120 can be much thicker, and thethickness 125 may be in a range from about 400 angstroms to about 500angstroms. In other alternative embodiments, the sacrificial layer 120may include polysilicon or an organic material doped with silicon.

The sacrificial layer 120 will be patterned into different portions in alater process, where each portion will be used as a mask for patterningthe layers below. In other words, the sacrificial layer 120 will “fix”the CD size of a transistor device. This will be discussed in moredetail below. The sacrificial layer 120 will also have very high etchingselectivity with the anti-reflective layer 110 when the anti-reflectivelayer 110 is etched, so that the fixed CD size may be maintained duringthe etching.

A patterned photoresist layer 130 is formed over the sacrificial layer120. The patterned photoresist layer 130 includes a plurality ofphotoresist portions 130A, 130B, and 130C. These photoresist portions130A-130C are formed by depositing a photoresist layer (for examplethrough a spin-coating process) over the sacrificial layer 120 and thenpatterning the photoresist layer with a photolithography process knownin the art, which may include one or more masking, exposing, baking,developing, and rinsing processes (not necessarily in that order).

The photoresist portions 130A-130C each have a thickness 135 that is ina range from about 800 angstroms to about 1500 angstroms. Thephotoresist portions 130A-130C also each have a width (lateraldimension) 145. The width 145 is set to be approximately equal to thedesired CD of a transistor device. As an example, the CD of a transistordevice may be its conductive channel length, which is approximately thewidth of its gate.

Referring now to FIG. 3, an etching process 160 (also referred to as apatterning process) is performed on the semiconductor device 40. Theetching process 160 is a dry etching process and uses afluorine-containing plasma as an etchant. In an embodiment, the etchantincludes CF₄ or CH₂F₂, or a combination thereof. A helium gas may beused as a carrying gas. The etching process 160 may have a process flowrate in a range from about 10 standard cubic centimeter per minute(sccm) to about 200 sccm. The etching process 160 may also have aprocess pressure in a range from about 1 mili Torr (mT) to about 10 mT,and a process power in a range from about 10 Watts to about 1000 Watts.

The photoresist portions 130A-130C serve as etching masks during theetching process 160, and unprotected portions of the sacrificial layer120 are etched away. The remaining portions of the sacrificial layer 120now become sacrificial components 120A, 120B, and 120C. The sacrificialcomponents 120A-120C have substantially the same width 145 as thephotoresist portions 130A-130C above.

In the embodiment where the sacrificial layer 120 includes a thindielectric material, a trimming process may be optionally performed tothe sacrificial components 120A-120C to fine tune their width 145. Thetrimming process may include an additional etching process to furthershrink the width 145. In any case, the CD of the transistor device issubstantially fixed at this stage, which is equal to the width 145.Therefore, it may be said that one of the functions of the sacrificiallayer 120 is that it defines the CD of a transistor, or that it is aCD-defining layer.

Referring now to FIG. 4, the photoresist portions 130A-130C are removedusing a photoresist removal process known in the art, such as astripping process or an ashing process. Thereafter, an etching process170 (also referred to as a patterning process) is performed on thesemiconductor device 40. The etching process 170 is a dry etchingprocess and uses a highly passivated etchant gas with a heavy molecularweight component. The molecular weight may be in a range from about 32to about 96. In an embodiment, the etchant of the etching process 170includes SO₂ and HeO₂ with a mixing ratio in a range from about 1:1 toabout 1:5. A process temperature at an Electrode-chuck is in a rangefrom about 40 degrees Celsius to about 70 degrees Celsius. In otherembodiments, the etchant may include CO₂, Cl₂, Ar, HBr, or an NR gas(which is a mixture of Ar and O₂). The etching process 170 may also havea process pressure in a range from about 1 mili Torr (mT) to about 10mT, and a process power in a range from about 10 Watts to about 1000Watts.

The sacrificial components 120A-120C serve as etching masks during theetching process 170, and unprotected portions of the anti-reflectivelayer 110 are etched away. The remaining portions of the anti-reflectivelayer 110 now become anti-reflective components 110A, 110B, and 110C.

The material compositions for the sacrificial layer 120 and theanti-reflective layer 110 are chosen such that they have an extremelyhigh etching selectivity (measured by ratios of etching rates) duringthe etching process 170. For example, the etching selectivity may behigher than about 10, and as high as about 100. Such high etchingselectivity helps ensure that the anti-reflective layer 110 retains thewidth 145 of the sacrificial components 120A-120C during the etchingprocess 170. The high etching selectivity also affords the sacrificialcomponents 120A-120C sufficient etching margin, meaning that thesacrificial components 120A-120C will not be completely consumed beforethe etching of the anti-reflective layer 110 is complete. Furthermore,the highly passivated heavy molecular weight etchant of the etchingprocess 170 helps keep the sidewalls of the etched anti-reflectivecomponents 110A-110C smooth and straight. In other words, undesiredlateral etching can be substantially prevented or reduced, thereby alsohelping the anti-reflective components 110A-110C to retain the width 145of the sacrificial components 120A-120C.

In addition, the high etching selectivity between the layers 120-110 andthe highly effective etchant of the etching process 170 make “linetwisting” defects much less likely to occur. Line twisting defects tendto occur when the stacks of etched layers and their etching masksthereabove become too tall, which may result in these stacks beingshifted or toppled during etching. Here, the high etching selectivitymeans that the sacrificial components 120A-120C need not be asthick/tall, and the highly passivated etchant substantially preventslateral etching, which both reduce the likelihood of line twistingduring etching.

From the above discussions, it can be seen that the present disclosureinvolves using separate layers to address the topography variation issueand the CD-defining issue. More specifically, the anti-reflective layer110 is used to address the topography variation issue through a dilutingeffect (as a result of having increased thickness), and the sacrificiallayer 120 is used to fix the CD size of a transistor. Since each ofthese layers 110 and 120 is designed and implemented for their ownspecific purposes, both the topography variation and the CD-definitionissues can be addressed very well. In comparison, many traditionalfabrication processes attempt to use the same layer to both solve thetopography variation issue and to define a CD size of a transistor.Consequently, neither of these issues are adequately addressed undertraditional fabrication processes.

Referring now to FIG. 5, an etching process 180 (also referred to as apatterning process) is performed on the semiconductor device 40. Theetching process 180 is a dry etching process and uses afluorine-containing plasma as an etchant. In one embodiment, the etchantincludes CF₃ and He with about a 1:2 ratio. In another embodiment, theetchant includes CF₄/CHF₃. The etching process 180 has a temperature atan Electrode-chuck that is in a range from about 40 degrees Celsius toabout 70 degrees Celsius. The etching process 180 may also have aprocess pressure in a range from about 1 mili Torr (mT) to about 10 mT,and a process power in a range from about 10 Watts to about 1000 Watts.

The sacrificial components 120A-120C and the anti-reflective components110A-110C serve as etching masks during the etching process 180, andunprotected portions of the hard mask layer 100 are etched away. Theremain portions of the hard mask layer 100 now become hard maskcomponents 100A, 100B, and 100C.

Also, as the etching process 180 is performed, the sacrificialcomponents 120A-120C are etched away, and the anti-reflective components110A-110C are substantially etched away or consumed as well. In anembodiment, all of the sacrificial components 120A-120C, and about ⅓ toabout ⅔ of the anti-reflective components are substantially etched awayby the time the etching process 180 is finished. Here, the relativelyhigh thickness of the anti-reflective layer 110 helps maintain asufficient etching margin during the etching process 180.

In the embodiment where the sacrificial layer 120 includes an organicmaterial, the hard mask components 100A-100C can be trimmed using atrimming process similar to that described above. The trimming processfine tunes the width of the hard mask components 100A-100C, which shouldbe slightly less than the width 145 after the trimming. In any case, thewidth of the hard mask components 100A-100C is used to set the CD sizefor a transistor device below, regardless if that width is substantiallyequal to the width 145 of the sacrificial components in one embodiment(where the sacrificial layer 120 includes a thin dielectric material),or slightly less than the width 145 of the sacrificial components inanother embodiment (where the sacrificial layer 120 include an organicmaterial).

Referring now to FIG. 6, the remaining portions of the anti-reflectivecomponents 110A-110C are removed, and an etching process 190 (alsoreferred to as a patterning process) is performed on the semiconductordevice 40. Using the hard mask components 100A-100C as etching masks,unprotected portions of the polysilicon layer 90 are etched away. Theremaining portions of the polysilicon layer 90 now become polysiliconcomponents 90A, 90B, and 90C. The polysilicon components 90A-90C alsoserve as dummy gate electrodes in a gate-last or a gate replacementprocess. Therefore, the polysilicon components 90A-90C may also bereferred to as dummy polysilicon gate electrodes 90A-90C. These dummypolysilicon gate electrodes 90A-90C each have the width 145, which willbe substantially equal to the channel length (CD) in the embodimentshown.

Had the topography variation issue not been properly addressed by thethick anti-reflective layer 110 and the CD size not been fixed by thesacrificial layer 120, the dummy polysilicon gate electrode 90B may havea width that is smaller than the other dummy polysilicon gate electrodes90A and 90C, because the isolation structure 50 is taller than otherareas of the substrate 45. Thus, CD uniformity would not have beenachieved, thereby leading to problems such as gate filling (describedlater) and/or variations in transistor currents (drain currents) andvoltages (threshold voltages). Here, the use of the thickanti-reflective layer 110 and the sacrificial layer 120 sufficientlyaddress these issues discussed above, and therefore the dummypolysilicon gate electrode 90B has substantially the same size as theother dummy polysilicon gate electrodes 90A and 90C. In other words, CDuniformity is much improved.

The conductive layer 80 and the high-k dielectric layer 70 are alsoetched using the hard mask components 100A-100C as etching masks,thereby forming conductive components 80A-80C and high-k dielectriccomponents 70A-70C underneath the dummy polysilicon gate electrodecomponents 90A-90C.

Referring now to FIG. 7, lightly doped source/drain regions 200A-201Aare formed in the doped well 60 and on opposite sides of the dummypolysilicon gate electrode 90A, and lightly doped source/drain regions200C-201C are formed in the doped well 61 and on opposite sides of thedummy polysilicon gate electrode 90C. The lightly doped source/drainregions 200A-201A and 200B-201B are formed using an ion implantationprocess or a diffusion process known in the art. No lightly dopedsource/drain regions are formed below the dummy polysilicon gateelectrode 90B.

Thereafter, gate spacers 210A and 211A are formed on sidewalls of thedummy polysilicon gate electrode 90A, gate spacers 210B and 211B areformed on sidewalls of the dummy polysilicon gate electrode 90B, andgate spacers 210C and 211C are formed on sidewalls of the dummypolysilicon gate electrode 90C. The gate spacers 210A-210C and 211A-211Care formed using a deposition process and an etching process (forexample, an anisotropic etching process) known in the art. The gatespacers 210A-210C and 211A-211C include a suitable dielectric materialsuch as silicon nitride, silicon oxide, silicon carbide, siliconoxy-nitride, or combinations thereof.

Thereafter, heavily doped source/drain regions 220A-221A are formed inthe doped well 60 and on opposite sides of the dummy polysilicon gateelectrode 90A, and heavily doped source/drain regions 220C-221C areformed in the doped well 61 and on opposite sides of the dummypolysilicon gate electrode 90C. The heavily doped source/drain regions220A-221A and 220C-221C are formed using an ion implantation process ora diffusion process known in the art. The heavily doped source/drainregions 220A-221A and 220C-221C have heavier dopant concentrations thanthe lightly doped source/drain regions 200A-201A and 200C-201C. Noheavily doped source/drain regions are formed below the dummypolysilicon gate electrode 90B. Since the dopants cannot penetratethrough the spacers 210A-210C and 211A-211C, the heavily dopedsource/drain regions 220A, 221A, 220C, and 221C are formed to beself-aligned with the spacers 210A, 211A, 210C, and 211C, respectively.

Referring now to FIG. 8, an inter-layer (or inter-level) dielectric(ILD) layer 250 is formed over the substrate 45. The ILD layer 250 maybe formed by CVD, high density plasma CVD, spin-on, sputtering, or othersuitable methods. In an embodiment, the ILD layer 250 includes siliconoxide. In other embodiments, the ILD layer 250 may include siliconoxy-nitride, silicon nitride, or a low-k material. The ILD layer 250 isformed to surround the dummy polysilicon gate electrodes 90A-90C as wellas the gate spacers 210A-210C and 211A-211C.

Thereafter, a chemical-mechanical-polishing (CMP) process is performedon the ILD layer 250 to flatten the top surface and to expose the dummypolysilicon gate electrodes 90A-90C. Following the CMP process, the topsurfaces of the dummy polysilicon gate electrodes 90A-90C aresubstantially co-planar with the top surface of the ILD layer 250.Although not illustrated, one or more annealing processes are performedon the semiconductor device 40 to activate the source and drain regions.These annealing processes may be performed before or after the CMPprocess.

Referring now to FIG. 9, the dummy polysilicon gate electrodes 90A-90Care removed, thereby forming openings (or trenches) 280A-280C in placeof the dummy polysilicon gate electrodes 90A-90C, respectively. Thedummy polysilicon gate electrodes 90A-90C and the conductive components80A-80C therebelow may be removed in a wet etching or a dry etchingprocess known in the art, while the rest of the layers of thesemiconductor device 40 remain substantially un-etched, including thegate spacers 210A-210C and 211A-211C and the ILD layer 250. This isperformed in accordance with a “gate last” process. Note that theopenings or trenches 280A-280C still maintain the width 145, which issubstantially equal to the critical dimension.

Referring to FIG. 10, metal gate electrodes 300A-300C are formed withinthe trenches 280A-280C, respectively, and over the high-k gatedielectric layers 70A-70C. The metal gate electrodes 300A-300C eachinclude a work function metal, which may be N-type and includes Ti, Al,Ta, ZrSi₂, or TaN, or P-type and includes Mo, Ru, Ir, Pt, PtSi, MoN, orWNx. The work function metal has a respective range of work functionsvalues associated therein. The work function metal tunes a work functionof its respective transistor so that a desired threshold V_(t) voltageis achieved. The metal gate electrodes 300A-300C also each include afill metal portion that serves as the main conductive portion of thegate electrode. The fill metal portions may include tungsten, aluminum,copper, or combinations thereof. For the sake of simplicity, the workfunction metals and the fill metal portions are not separately drawn.

Described above is a gate-last process flow. It is understood that thevarious aspects of the present disclosure can also be applied to ahigh-k last process flow. In that case, an oxide dielectric layer isformed in place of the high-k dielectric layer 70 originally. The oxidedielectric layer is removed along with the dummy poly gate electrodes300A-300C. A high-k gate dielectric layer is then formed in the openings280A-280C before the metal gate electrodes are formed in these openings.In both the gate-last process flow and the high-k last process flow, thegate electrodes 300A-300C all have substantially equal widths orcritical dimensions. In other words, very good CD uniformity can beachieved.

Therefore, the embodiments of the present disclosure offers advantages,it being understood that different embodiments may offer differentadvantages, and that no particular advantage is required for allembodiments. One of the other advantages is that both the topographyvariation issue and the CD-definition issue can be sufficientlyaddressed. As discussed above, many traditional fabrication methodsattempt to use a single layer to resolve both the topography variationissue and the CD-definition issue. As a result, neither of these issuesis adequately resolved.

In comparison, the present disclosure uses two separate layers to dealwith these two issues. A thick anti-reflective layer 110 is used todilute the topography variation effects (such as step height).Meanwhile, a sacrificial layer 120 having a very high etchingselectivity with the anti-reflective layer 110 is used to fix the CDsize. Therefore, the CD size can be accurately defined, and thetopography variation effects can be minimized at the same time. Also,the use of the highly passivated etchant gas with the heavy molecularweight component reduces undesired lateral etching and keeps thesidewall profile of the anti-reflective layer 110 smooth and straightwhile it is etched. This also helps the CD uniformity.

Having good CD uniformity may be advantageous for the gate-replacementprocess flow described above. In the gate-replacement process, the dummypolysilicon gate electrodes are removed to form openings, and theopenings are filled with metal gate electrodes. If CD uniformity is notachieved, then some of these openings (such as an opening located abovean STI device) may become too small to be filled, or will be filled butmay contain air bubbles. This is particularly true as device sizesbecome smaller and smaller. Here, substantial CD uniformity is achievedin spite of topography variations. Consequently, filling the gateopenings with metal is not a problem. Furthermore, good CD uniformityobtained by the present disclosure can help minimize undesiredvariations of drain currents or threshold voltages among differenttransistor devices.

Furthermore, the present disclosure can be easily integrated intoexisting process flow. Therefore, it does not increase fabricationcosts. Also, the present disclosure involve using fewer layers thantraditional processes, therefore it saves process time accordingly. Inaddition, the heavy molecular weight etchant used in the presentdisclosure (such as SO₂) has a very fast etching rate. As an example, itmay only take about 15-20 seconds to “open” (or etch through) theanti-reflective layer 110 with a thickness of about 2000 angstroms. Incomparison, opening another anti-reflective layer with a traditionaletchant may take about 100 seconds. Overall, the present disclosure maysave total processing time by about 40-50%. Since processing time isdirectly related to fabrication costs, the total fabrication costs maybe substantially reduced by implementing the processes and materialstaught in the present disclosure.

It is understood that additional processes may be performed to completethe fabrication of the semiconductor device 40. For example, theseadditional processes may include formation of interconnect structures(e.g., lines and vias, metal layers, and interlayer dielectric thatprovide electrical interconnection to the device including the formedmetal gate), deposition of passivation layers, packaging, and dicing.For the sake of simplicity, these additional processes are not describedherein.

One of the broader forms of the present disclosure involves a method offabricating a semiconductor device. The method includes: forming apatternable layer over a substrate; forming a first layer over thepatternable layer; forming a second layer over the first layer, thesecond layer being substantially thinner than the first layer;patterning the second layer with a photoresist mask through a firstetching process to form a patterned second layer; patterning the firstlayer with the patterned second layer through a second etching processto form a patterned first layer, wherein the first and second layershave substantially different etching rates during the second etchingprocess; and patterning the patternable layer with the patterned firstlayer through a third etching process.

Another of the broader forms of the present disclosure involves a methodof fabricating a semiconductor device. The method includes: forming ahard mask layer over a substrate; forming an anti-reflective layer overthe hard mask layer, the anti-reflective layer having a first thickness;forming a sacrificial layer over the anti-reflective layer, thesacrificial layer including one of: a dielectric material having asecond thickness, and a nitrogen-containing organic material having athird thickness, wherein the second and third thicknesses are bothsubstantially smaller than the first thickness, and wherein a ratio ofthe first thickness to the second thickness is several times greaterthan a ratio of the first thickness to the third thickness; forming apatterned photoresist layer over the sacrificial layer; performing afirst etching process on the sacrificial layer using the patternedphotoresist layer as a first etching mask, thereby forming a patternedsacrificial layer; performing a second etching process on theanti-reflective layer using the patterned sacrificial layer as a secondetching mask, thereby forming a patterned anti-reflective layer, whereinan etching selectivity between the sacrificial layer and theanti-reflective layer with respect to the second etching process isgreater than about 10; and patterning the hard mask layer with thepatterned anti-reflective layer to form a patterned hard mask layer.

Still another of the broader forms of the present disclosure involves amethod of fabricating a semiconductor device. The method includes:forming a polysilicon layer over a substrate; forming a hard mask layerover the polysilicon layer; forming an anti-reflective layer over thehard mask layer, the anti-reflective layer containing carbon andhydrogen, wherein a carbon content is greater than a hydrogen content byat least about 10 times; forming a sacrificial layer over theanti-reflective layer, the sacrificial layer being substantially thinnerthan the first layer, the sacrificial layer containing one of: adielectric material and a nitrogen-containing organic material;performing a first patterning process on the sacrificial layer using aphotoresist mask, the first patterning process using afluorine-containing plasma as an etchant; performing a second patterningprocess on the anti-reflective layer using the patterned sacrificiallayer, the second patterning process using an etchant gas that includesa component with a molecular weight in a range from about 32 to about96, wherein the anti-reflective layer and the sacrificial layer havesubstantially different etching rates during the second patterningprocess; and performing a third patterning process on the hard masklayer using the patterned anti-reflective layer; and performing a fourthpatterning process on the polysilicon layer using the patterned hardmask layer.

The foregoing has outlined features of several embodiments so that thoseskilled in the art may better understand the detailed description thatfollows. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions andalterations herein without departing from the spirit and scope of thepresent disclosure. For example, the high voltage device may not belimited to an NMOS device and can be extended to a PMOS device with asimilar structure and configuration except that all doping types may bereversed and dimensions are modified according to PMOS design. Further,the PMOS device may be disposed in a deep n-well pocket for isolatingthe device.

1. A method of fabricating a semiconductor device, comprising: forming apatternable layer over a substrate, wherein the patternable layerincludes a polysilicon gate electrode layer; forming a first layer overthe patternable layer; forming a second layer over the first layer, thesecond layer being substantially thinner than the first layer;patterning the second layer with a photoresist mask through a firstetching process to form a patterned second layer; patterning the firstlayer with the patterned second layer through a second etching processto form a patterned first layer, wherein the first and second layershave substantially different etching rates during the second etchingprocess; and patterning the patternable layer with the patterned firstlayer through a third etching process.
 2. The method of claim 1, whereinthe patterned second layer includes a plurality of components, each ofthese components having a dimension that defines a critical dimension.3. The method of claim 2, further including performing a trimmingprocess to the second layer to fine tune the critical dimension.
 4. Themethod of claim 1, wherein the first layer is at least about ten timesthicker than a variation in topography of the substrate.
 5. The methodof claim 1, wherein: the second layer is several times thinner than thefirst layer; and the etching rates between the first and second layersdiffer by at least ten times.
 6. The method of claim 1, wherein: thepatternable layer includes a hard mask material; the first layerincludes an anti-reflective material; and the second layer includes oneof: a dielectric material, and a nitrogen-containing organic material.7. The method of claim 6, the first layer contains both carbon andhydrogen, and wherein: the carbon concentration is at least ten timeshigher than the hydrogen concentration; a refractive index of the firstlayer is greater than about 1; and an extinction coefficient of thefirst layer is lower than about 0.5.
 8. The method of claim 1, whereinthe second etching process is a dry etching process and uses an etchantthat contains SO₂.
 9. A method of fabricating a semiconductor device,comprising: forming a hard mask layer over a substrate; forming ananti-reflective layer over the hard mask layer, the anti-reflectivelayer having a first thickness; forming a sacrificial layer over theanti-reflective layer, the sacrificial layer including one of adielectric material having a second thickness, and a nitrogen-containingorganic material having a third thickness, wherein the second and thirdthicknesses are both substantially smaller than the first thickness, andwherein a ratio of the first thickness to the second thickness isseveral times greater than a ratio of the first thickness to the thirdthickness; forming a patterned photoresist layer over the sacrificiallayer; performing a first etching process on the sacrificial layer usingthe patterned photoresist layer as a first etching mask, thereby forminga patterned sacrificial layer; performing a second etching process onthe anti-reflective layer using the patterned sacrificial layer as asecond etching mask, thereby forming a patterned anti-reflective layer,wherein an etching selectivity between the sacrificial layer and theanti-reflective layer with respect to the second etching process isgreater than about 10; and patterning the hard mask layer with thepatterned anti-reflective layer to form a patterned hard mask layer. 10.The method of claim 9, wherein: the first thickness is in a range fromabout 1000 angstroms to about 2000 angstroms; the second thickness is ina range from about 50 angstroms to about 200 angstroms; and the thirdthickness is in a range from about 400 angstroms to about 500 angstroms.11. The method of claim 9, wherein the anti-reflective layer has acarbon concentration in a range from about 65% to about 85%, and ahydrogen concentration in a range from about 3% to about 6.5%, arefractive index in a range from about 1.4 to about 1.7, and anextinction coefficient in a range from about 0.2 to about 0.4.
 12. Themethod of claim 9, wherein the second etching process is a dry etchingprocess and uses an etchant having a heavy molecular weight componentwith a molecular weight being in a range from about 32 to about
 96. 13.The method of claim 9, wherein the second etching process uses anetchant that includes SO₂ and HeO₂ with a mixing ratio in a range fromabout 1:1 to about 1:5.
 14. The method of claim 9, wherein the patternedsacrificial layer includes at least one component whose size defines acritical dimension of a transistor.
 15. The method of claim 9, furtherincluding: before the forming the hard mask layer, forming a dielectriclayer over the substrate and forming a polysilicon layer over thedielectric layer; patterning the polysilicon layer with the patternedhard mask layer to form dummy polysilicon electrodes; removing the dummypolysilicon electrodes, thereby forming openings in place of the dummypolysilicon electrodes; and thereafter filling the openings with metalelectrodes.
 16. A method of fabricating a semiconductor device,comprising: forming a polysilicon layer over a substrate; forming a hardmask layer over the polysilicon layer; forming an anti-reflective layerover the hard mask layer, the anti-reflective layer containing carbonand hydrogen, wherein a carbon content is greater than a hydrogencontent by at least about 10 times; forming a sacrificial layer over theanti-reflective layer, the sacrificial layer being substantially thinnerthan the first layer, the sacrificial layer containing one of: adielectric material and a nitrogen-containing organic material;performing a first patterning process on the sacrificial layer using aphotoresist mask, the first patterning process using afluorine-containing plasma as an etchant; performing a second patterningprocess on the anti-reflective layer using the patterned sacrificiallayer, the second patterning process using an etchant gas that includesa component with a molecular weight in a range from about 32 to about96, wherein the anti-reflective layer and the sacrificial layer havesubstantially different etching rates during the second patterningprocess; and performing a third patterning process on the hard masklayer using the patterned anti-reflective layer; and performing a fourthpatterning process on the polysilicon layer using the patterned hardmask layer.
 17. The method of claim 16, wherein the polysilicon layer ispatterned into dummy gate electrodes by the fourth patterning process,and further including: before the forming the polysilicon layer, forminga gate dielectric layer over the substrate; forming doped source/drainregions in the substrate on opposite sides of the dummy gate electrodes;forming an inter-layer dielectric (ILD) structure over the substrate,the ILD structure at least partially surrounding the dummy gateelectrodes; and forming openings in the ILD structure by removing thedummy gate electrodes; and thereafter filling the openings with metalgate electrodes.
 18. The method of claim 16, wherein: theanti-reflective layer has a thickness that is in a range from about 1000angstroms to about 2000 angstroms; if the sacrificial layer includes thedielectric material, a thickness of the sacrificial layer is in a rangefrom about 50 angstroms to about 200 angstroms; and if the sacrificiallayer includes the nitrogen-containing organic material, a thickness ofthe sacrificial layer is in a range from about 400 angstroms to about500 angstroms.
 19. The method of claim 16, wherein: the carbon contentand the hydrogen content of the anti-reflective layer are measured withrespect to weight; and the anti-reflective layer has: a molecular weightthat is in a range from about 4000 to about 5000, a refractive indexthat is greater than about 1, and an extinction coefficient that is lessthan about 0.5, the refractive index and the extinction coefficientbeing measured with respect to a 193 nanometer process.
 20. The methodof claim 16, wherein: the first and third patterning processes each usea fluorine-containing plasma as an etchant; and the second patterningprocess uses an etchant that includes SO₂ and HeO₂, a ratio of the SO₂to the HeO₂ being less than about 1:1.